Capture partial reborrows of &mut aggregate fields in mut_locals

src/analyze/local_def.rs

@@ -649,13 +649,15 @@ impl<'tcx, 'ctx> Analyzer<'tcx, 'ctx> {
             }
 
             fn visit_operand(&mut self, operand: &mir::Operand<'tcx>, location: mir::Location) {
-                if let mir::Operand::Move(place) = operand {
-                    // to be reborrowed; see analyze::basic_block::visitor
-                    if place
-                        .ty(&self.body.local_decls, self.tcx)
-                        .ty
-                        .is_mutable_ptr()
-                    {
+                // to be reborrowed; see analyze::basic_block::visitor. A `&mut`
+                // field read out of an aggregate is `copy (_1.0)`, so match Copy
+                // too to mark the base local. Reference only: `is_mutable_ptr()`
+                // also covers `*mut`, which is Copy but never reborrowed.
+                if let mir::Operand::Move(place) | mir::Operand::Copy(place) = operand {
+                    if matches!(
+                        place.ty(&self.body.local_decls, self.tcx).ty.kind(),
+                        mir_ty::TyKind::Ref(_, _, m) if m.is_mut()
+                    ) {
                         self.locals.insert(place.local);
                     }
                 }

tests/ui/fail/reborrow_mut_field_of_aggregate_param.rs

@@ -0,0 +1,18 @@
+//@error-in-other-file: Unsat
+
+// Regression test for #125: reborrowing a `&mut` field out of an aggregate
+// parameter used to panic with "deref unbound var".
+
+fn bump(r: &mut i64) {
+    *r = 1;
+}
+
+fn f(w: (&mut i64,)) {
+    bump(w.0);
+}
+
+fn main() {
+    let mut x = 0_i64;
+    f((&mut x,));
+    assert!(x == 0);
+}

tests/ui/pass/reborrow_mut_field_of_aggregate_param.rs

@@ -0,0 +1,18 @@
+//@check-pass
+
+// Regression test for #125: reborrowing a `&mut` field out of an aggregate
+// parameter used to panic with "deref unbound var".
+
+fn bump(r: &mut i64) {
+    *r = 1;
+}
+
+fn f(w: (&mut i64,)) {
+    bump(w.0);
+}
+
+fn main() {
+    let mut x = 0_i64;
+    f((&mut x,));
+    assert!(x == 1);
+}